An air turbine starter is a device used to start a turbine engine, such as a gas turbine jet engine commonly found on aircraft. The air turbine starter is connected to the jet engine and is used to start the jet engine in generally the same way as a starter for an automobile is used to start the automobile's engine. The developer of the present inventions, Honeywell International, Inc., has for years successfully designed, developed, manufactured and repaired air turbine starters.
FIG. 1 shows a partial cut-away diagram of a conventional air turbine starter 100, having a first housing 103 which includes an air inlet 105, air outlets 107, and a second housing 111 including a turbine assembly 113, gearbox 115, and starter output shaft, not shown. The turbine assembly 113 has a turbine wheel 117 with circumferentially mounted blades 119, a rotatable drive shaft 121, and gear 123. Additional turbine starter features are disclosed in Honeywell's U.S. Pat. No. 6,318,958 (Giesler et al.) and U.S. Pat. No. 4,914,906 (Burch) which is incorporated by reference herein.
In order to start the jet engine the air turbine starter is activated. Generally speaking, activation is accomplished by connecting an air pressure duct to air inlet 105 to supply compressed air. Compressed air is directed by the contour passages of the first housing 105 through the blades 119 and is vented from air outlets 107. In operation, the energy of the moving air is converted by blades 119 into rotary motion, causing the turbine assembly 113 to rotate.
Stability of the turbine assembly 113 during starting and engine driving operations is important to the operational longevity of the air turbine starter 100. As shown in FIG. 2, an angular contact thrust bearing 201 is used to support and stabilize the turbine drive shaft 121. The angular contact thrust bearing 201 supports drive shaft 121 and allows it to rotate without much friction losses. As shown, in FIGS. 2 and 2A, the angular contact thrust bearing 201 has an outer ring 203, an inner ring 205, a cage 207, and balls 209.
During the process of starting the jet engine or driving the jet engine without starting as may be required for maintenance, significant forces are applied to the angular contact thrust bearing 201. Oil lubrication is supplied to the angular contact thrust bearing 201 as represented by arrow 211 in FIG. 2 and arrow 134 in FIG. 1 by an internal oil circulation system.
Bearing wear is understood to be a natural result of bearing operation. In particular, foreign matter in the oil or bearing components, heat and stress from use, and many other factors can cause bearing wear and degrade bearing performance. As shown in FIG. 2A, hard debris 213 may become lodged as a clump 215 in the angular contact bearing 201 between the cage 207 and outer ring 203. These debris may be from the manufacturing process (swarf, chips, or grit), from internal components (wear debris or chemical attack of bearing surfaces), from the outside environment (such as sand or dust), from lubricant breakdown (sludge or precipitates), or other sources.
Further, the debris 213 may become lodged in and or deteriorate the race surface 217 (also commonly known as a raceway) 217 and cause significant imperfections 219 in the race surface 217 of the outer ring 203. In a similar fashion, the debris 213 may cause significant imperfections 233 in the race surface 235 of the inner ring 205. Typically, the imperfections 219 and 223 occur in the regions of the race surfaces 217 and 235 where the force of thrust, shown as arrows 237, transfers between inner ring 205, ball 209 and outer ring 203.
In addition, debris 213 may cause imperfections such as gouges 221 or scratches in the surface of the ball 209. More specifically, debris 213 may lead to the breakdown of the lubricating film between the ball 209 and race surfaces 217 and 235, leading to the eventual spalling of the raceway and ball 209 materials—events which lead to shortened operational life of the bearing.
Proper operation of angular contact thrust bearing 201 is desired for proper operation of the starter. Should the angular contact thrust bearing 201 become seized or unduly worn, damage to the turbine assembly and or gearbox may result. In such an event a more extensive rebuild of the starter beyond mere bearing replacement may be needed. This may be costly and time consuming. However, it should be appreciated that despite this drawback, air turbine starters are generally operationally safe and reliable. Routine maintenance, including bearing inspection and replacement, is a normal periodic event based in part on the expected operational life of the angular contact thrust bearing 201.
Hence, there is a need in for an improved air turbine starter having a longer life bearing with improved characteristics to overcome one or more of the drawbacks identified above. The present invention satisfies one or more of these needs.